Recently got my Nexus 5 in the mail, after looking at some people's successes with putting it on MetroPCS. Went to a kiosk at the local mall, where they then activated it, and made a test call, which worked. They forgot to check internet connectivity, and I don't think that I had it at the time. Anyway...
I left there, and voila! no signal. I took it to a standalone, but private, Metro store, and he fiddled with it, and it suddenly started working, hspa and all! it worked for the remainder of the night, and the next morning, no signal. (please note, when it works, the apn settings list metro, tmobile, and a blank tmobile entry, with the metro one selected)
Currently, reboots, cycling airplane mode, turning off and on 3g, and trying to manually configure apns don't work.
Please help, I'm very frustrated.
(P.S. When it doesn't work, the phone refuses to save a manually entered APN)
I left there, and voila! no signal. I took it to a standalone, but private, Metro store, and he fiddled with it, and it suddenly started working, hspa and all! it worked for the remainder of the night, and the next morning, no signal. (please note, when it works, the apn settings list metro, tmobile, and a blank tmobile entry, with the metro one selected)
Currently, reboots, cycling airplane mode, turning off and on 3g, and trying to manually configure apns don't work.
Please help, I'm very frustrated.
(P.S. When it doesn't work, the phone refuses to save a manually entered APN)
anyway within two hours of activation i was with the metro customer service to complain about the exact problem you were describing, it dropped the call three times, never could finish the complaint. 
then i called from my boost phone and eventually had it done. they made me reset the phone including taking the sim card out etc. so it is better to call from another phone. its also is good idea for you to give them your imei number from settings/about phone and sim card number from your activation receipt. to make sure they have it in the system.