How to avoid vodafone roaming charges ?

Hi ,
I am shifting from Punjab to Mumbai . I don't want to change my vodafone mobile number as it is associated with most of my official documents. what roaming charges can I expect on vodafone ?
Even If get a new sim card , I have to change it every time as I don't own a dual sim phone.(iPhone 4)
I don't want to get a new dual sim phone either. (too expensive for me)
Please give suggestions .
Thanks in advance

National MNP

Well , that is a very nice option for me . However, I might travel to Punjab in-between for official purposes . I think a new cheap phone and another sim might work for me . Thanks for replying .

vodafone punjab roaming pack
31rs free incoming 30days validity
74rs free incoming 90days validity
outgoing call rate is standard in both the packs.

My experience on roaming in NCR was shocking, my vodafone SIM from U.P.(East) has network issues, other SIM from RIM (GSM) U.P(East) was even more horrible,
I ported my Reliance to Airtel NCR, while voda one is still on roaming, I use only 3G, and number is only with family members
I wil port this too, yet to visit a Vodafone store to PORT.

Things to remember while you roam on a different region number :-
1). you might not get calls, if network is weak, I think roaming numbers are least preferred on network as of now.
2). You need to top up a roaming pack, else heavy incoming charges.
3). No Free calls based on any pack and you would end up paying heavy outgoing charges.
4). No Free SMS or SMS pack would work.
5). No special offers or packs as you are on "Paraya Network" Lol

so if you have moved to a new region/State, its better to PORT

For low uasge roaming rates are not that bad, like I pay 2p/sec flat i. e. Rs. 1.2/min for all calls while if i use roaming sim it would be 80p/min for local 1.15/min for std.
sms charges have even more difference. 28p roaming vs Re. 1local.
only diff is incoming but again not much big for low usage user.

There are other benfits of NMNP,
if you have moved to another region and keep using your OLD number on roaming, and you loose the SIM/Phone
you would need to get the new SIM from same State/region where it was issued from, so Its better to carry a number belonging to
region you stay the most.

I am porting 2 more numbers from U.P (East) to NCR today cause can't afford to loose that number.

@ghpk

RCOM GSM + Aircel works pretty good in NCR! Though, you need a local NCR number to be able to access Aircel ICRA, afaik.

Don't know why, Rcom gave me a few shocks.
Network used to disappear and in many instances I got busy tone while dialing out,
my caller complained about Not Reachable even when I had signal bars.

Very bad network quality and phone calls were in robotic voice, so Ported out.