Bought this phone on 2-year contract from a Virgin Mobile booth.
On a side note, it’s not advertised as “601”, but I figured this one is closest to the specs of the phone being sold in Canada.
Anyway, starting day one, this phone was showing wireless signal problems (mobile network). I live in the GTA, your experiences may vary.

I used RF Signal Tracker to investigate.

Note

RF signal tracker is not very stable for monitoring purposes, so for best results leave your phone open/unlocked with screen on.
If it does not start monitoring, you may need to clean the cache, install/uninstall or a combination thereof.

Experiment

Two phones were monitored during the same time frame, for roughly 1 hour. Another phone was Samsung Galaxy Ace II.
This HTC Desire was dropping a network completely every 20min or so, appearing as if there was no sim card for a brief 15 seconds and then going back to normal.
The other phone was showing no signal issues (almost constant signal level in dB).

Here follows a lot of research, which led nowhere, that’s why I am not gonna post any of it. :)

Last thing, I decided to call Virgin Mobile support, and they suggested I should try playing with network mode, mentioning that 4G is generally more stable.
I went to the settings, and guess what - it was already set to 4G (auto). So I put it on 3G (auto) to see what happens, and - it solved the problem (confirmed with RF signal tracker app).
It would probably have worked just fine on “GSM only”, but I copied this setting from my Samsung Galaxy S2, which never had these network drops.

And here is how you do it:

Settings -> Mobile data -> Network mode -> Set "Preferred network mode" to "GSM / WCDMA auto".

P.S. Almost immediately upon receipt, I got this phone upgraded from Android 4.2.2 (HTC Sense v5.0) to Android 4.4.2 (Sense v5.5).
This has no impact on the original issue, just mentioning for the sake of completeness (also to prove it supports 4.4.x, if anyone is looking for confirmation).