There is a shortage of Hub5's so they are giving them to "new" 1GB customers as a priority over "upgraders".
The reasoning is that you have a fully working 1GB service with the Hub4 so dont need it as the 2.5GB port isnt "essential" for the 1 GB service- your are getting the speeds guaranteed in the T&C's.
One or two threads have been appearing where some Hub4 customers are receiving Hub5's now so the stock situation may be easing and you may get lucky - so keep trying.
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John
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I do not work for VM. My services: HD TV on VIP (+ Sky Sports & Movies & BT sport), x3 V6 boxes (1 wired, 1 WiFi, 1 on PA) Hub5 in modem mode with Apple Airport Extreme Router +2 Airport Express's. On 250Mbps, Talk Anytime Phone, x2 Mobile SIM only iPhones.