Slow call pickup

Hi, we've got a MyPBX U100 using PSTN trunking (4 lines), and a total of 15 T22P VOIP phones. All of these phones are in the same call pickup group.

We've noticed two issues:
• When someone rings in on a phone, there's a delay of about 4 seconds before the T22P phone starts ringing.
• When we try to do a call pickup (*4 via the pickup group), there's also about a 4 second delay before the call is actually picked up.

Anyone know why this would be happening, and what to do about it?

Thanks