Sometimes I use IG Index for small caps.. I’ve noticed whenever something gets interesting; for example BAO last week it goes to phone dealing only. While selling BAO on the phone I asked the question.. The answer was it could mean lots of things but a prominent reason is the ability of IG to buy the shares as a hedge.
I think if it is tough for IG to trade themselves in order to hedge; then they might go to phone dealing only..
Note: In general providers offer the same spread whether you choose to bet online or over the phone. The two services complement each other – you can open a bet over the phone and close it online and vice versa.