This is a trait, a character feature that has both a benefit and a drawback. Similar arrangements with different flavors could easily be imagined for other planar races which have some benefit to offer summonable planewalkers.
Benefit: You have legal access to the Labyrinthine Portal of the modrons, and the knowledge of locations of portals it contains from Mechanus to other worlds. Modrons understand you to be permitted when encountered, and you may make Knowledge checks to find a good route as described under "Labyrinthine Portal Travel." You may bring small parties with you; passing pentadrones swiftly judge whether this clause is being abused.
Normal: Without this trait, individuals must petition the modron hierarchy for legal access to the Labyrinthine Portal for each trip, which usually comes with one set of accurate directions. This can take considerable time and difficulty, and is normally only granted for in-Mechanus trips. Failing to obtain legal access means pentadrone and hierarch modrons encountered along the way will attempt to hinder the traveler, and reliable information can be hard to come by.
Prerequisite: You must be lawful and of planar birth to take this trait. Typically, you must also be a member of a modron-approved organization.
Drawback: You have purchased this access with an agreement to come to the aid of modrons when necessary: not more than once every 17 days nor less than once every 219 days, you are summoned without notice as a summoned creature, under the command of a modron spellcaster of appropriate level. Because the spell usually seeks a combat-ready individual to fight for the summoner, you are usually plucked from active combat, or at least fully armed. The GM may simply decide that this happens at the start of a combat in the allowable time frame, or, if they prefer, on a natural initiative roll of 17 during this time. From your starting point, you seem to be hit by a tiny crystal suddenly appearing in your vicinity, and vanish. This effect never takes place within Sigil.
You appear near the spellcaster and have a general understanding of which being the caster is, and who their allies and enemies are, though not necessarily why. You act immediately in the round you are summoned. It is relatively rare for you to be summoned to an engagement on Mechanus, since hierarch modrons have access to many troops there, although on occasion some strong interloper must be dealt with. More often you will be summoned to support a hierarch modron performing off-Mechanus duties, vulnerable to attack. Your enemies will often be creatures of chaos, with at least as many HD as you have, that would be pleased to assassinate an officer of the modron race. (The overall CR of the encounter the modron is facing may be much greater than your ECL; you are just one cog in the modron's arsenal.) While a dead hierarch modron is immeediately replaced by a string of promotions, and its life energy returns to the communal energy pool on Regulus and creates a new monodrone to replace a monodrone at the bottom of the string, this promotion process does not preserve memory.
As a summoned creature no wounds or other lasting effects will last beyond the spell's duration, although you can use up spells and consumable items. (However, you may not summon or otherwise conjure another creature, nor use teleportation or planar travel abilities.) You will not be summoned into an environment which cannot support you. You can understand your summoner and are obliged to obey their commands for the duration of the spell, unless they would cost you XP or would cost someone XP if cast as a spell. (Typically, modrons will not bother to even ask something someone is not legally required to do; an offer to do so is permissible and may earn you some respect.) You may attempt to pervert the caster's intent if the commands would be strongly objectionable. Successfully twisting summoners' intent too often may result in revocation of this trait, however.
Your summoner is a hierarch modron with at least as many HD as you have, casting spells as a cleric of between 10th and 20th level. This level will be the duration of the spell in rounds, unless you are killed or dismissed. At the end of the spell's duration, you return to your starting position, or the nearest safe location if this has changed. If apparently killed, you reform in your previous state after 24 hours. It is unclear where you were in the meantime, though many planar sages would like to find out.
Labyrinthine Portal Travel:
When you wish to travel a long distance between cogs on Mechanus, you may make a Knowledge check (the Planes, or Local, as long as your specialties in either include Mechanus) to reduce travel time by calculating or remembering a useful route between convenient nodes of the Labyrinthine Portal. You can also attempt to travel to bordering Outer Planes, or to the Prime. These portals may not exit close to your destination, nor might they be permanently in the region they exit. Suggested check DCs for this travel are listed below, suitable for a campaign in which travel across planar regions often takes days or weeks, and reliable travel between planes is normally the mark of high-level characters.
Reduce travel time by 1 day between cogs(min. 1/2 day) 15
Reduce travel time by additional day (to min. 1/2) +5 each further day
Portal to bordering Outer Plane within week's march 20
Portal from bordering Outer Plane to Mechanus 25
(may be any distance)
Portal from Mechanus to particular Prime (any distance) 30
Portal from current Prime to Mechanus (any distance) 30
Portal more convenient to desired geographic location retry, +10 at least
(can only attempt if original roll was successful)
"Any distance" may be anywhere on the current plane and exiting anywhere on the target plane, possibly months of travel away from a desired destination without teleportation. Of course, they may also be relatively convenient.
GMs making these DCs higher or lower will make travel with the Portal more common or less. At the GM's discretion, checks for particular portals may be a great deal harder or easier; for example, Athas is almost wholly disconnected from the Outer Planes, and the DC to find a portal to Athas would be nearly impossible, while the Automata portal (to and from the Outlands) is extremely well known. Reliable maps that aid in these checks are rare (fake maps are fairly common), but spending time making observations with an etheroscope or astrological instruments (on a Prime), or gathering information from informed natives and casting guiding divinations, can provide bonuses to the check, often quite large and enough to find a convenient nearby portal.
The Difficulty Class of these checks for people without legal access to the Labyrinthine Portal is usually increased by at least +10, as some routes are guarded or secret. The modrons reserve the right to restrict access to any particular portion of the Portal at any time, whether one has this trait or not.