At this time, ForeFlight does not support sending text messages or making phone calls from the ForeFlight Mobile EFB app through a Garmin Flight Stream 210 or 510.
Of note, while not supported by ForeFlight, sending text messages or making phone calls utilizing your Flight Stream would also require having a GTN6XX/7XX Navigator (with GTN software version 6.20 and later), a GSR 56 Satellite transceiver, and a monthly service plan.