UCx50E, UCx450 and UCx1000 can include an FXO expansion card with up to 8 analog trunks (or a PRI card with up to 4 spans).
From the beginning of this year (or maybe late last year), E-MetroTel only ships UCx50E, which has this capability. In other words, for all new systems, you can order the desired PSTN trunks.
If you have the older UCx50 (not UCx50E), you'd have to use an analog trunk (FXO) gateway - for example Grandstream GXW4104, AudioCodes MP114 or similar.