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snaimon (Offline)
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Join Date: 17 Mar 2004
Location: Richmond, VA USA

Default 28-06-2006, 03:21

Originally Posted by DRNewcomb
When should I roam rather than buying a prepaid SIM?
"It is not a given that it is always cheaper to use a prepaid account, but it does not take many calls at $1.24+/min to make it cheaper. The formula, which I have developed (and humbly dubbed "Newcomb's Cypher" is:
Break_even_minutes = Bare_SIM_price/(Roaming_rate - Prepaid_rate)

Subtract the cost of a typical prepaid call from the cost of the same call while roaming. Divide the result into the cost of the prepaid SIM, discounting it by the value of any included minutes. For some combinations of roaming and prepaid you will find that it only takes a few minutes of use to pay for the prepaid."
Let's put some realistic #s to it AND correct me if I am wrong:

Prices in Euros

Start pack cost 20 with 10 Euro credit so raw cost is 10 Euros

Roaming rate say German SIM roaming in FR 1.59 Euros from documentation I have for an OUTGOING call from a D1 SIM.

Rate on FR SIM to FR .55 Euros


Break even minutes = 10 (bare SIM) / (1.59 - .55)
= 10 / 1.04
= 9 minutes

I take it the same can be done for INCOMING minutes

= 10 / (.79 - 0) (D1 inbound rate / minute is .79 Euros)
= ~ 13

So, in this case, if you think you will make or receive any more than 9 - 13 minutes of roaming calls while in FR with your German SIM, you would according to Newcomb's Cypher probably be better off buying the FR SIM.

BTW, the D1 highest roaming rate in in Zone 5, Egypt, Bulgaria, Thailand, Malta, Hong Kong, and a few others at 3.49. With that kind of outbound rate, the break even will be around 3 minutes in the above example.

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